Control of engine operation is accomplished by use of mechanical or electronic devices, which are commonly referred to as engine governors. An engine governor controls operation of the engine based on a condition or status of the engine. An engine governor, for example, may monitor and control the operation of the engine through a series of operating points, such as when a vehicle is accelerating and/or shifting gears. For gasoline or spark ignition engines, the engine governor controls engine speed by controlling a throttle valve or any other suitable device that modulates the air intake of the engine. For diesel or compression ignition engines, the engine governor may control operation of the engine based on a fuel or torque command to the engine. Regardless of the type of engine or governor being used, efficient control of the engine is desired during operation.
A typical application for automatic control of an engine during transient or changing conditions includes cruise control, such as what is used on vehicles to automatically adjust engine operation to maintain a constant vehicle speed. In other applications, such as in earthmoving equipment and other types of machines, automatic engine governing may be used to maintain a constant engine speed during work functions of the machine. Maintaining a constant engine speed in various applications is often a challenge. For example, an earthmoving machine may experience load changes during operation, which cause fluctuations in the load demanded by various work implements of the machine. Examples of such operation include a bucket loader operating to load material onto a truck, an excavator digging a hole, a bulldozer encountering an obstacle, and so forth. Fluctuations in load may directly affect engine speed.
In general, engine droop is a change of engine speed by the engine governor under certain operating conditions. In a typical application, engine droop is used in association with cruise control systems in vehicles. A cruise control system is a device that maintains a constant vehicle speed during travel by controlling engine speed. When a vehicle ascends a hill, the load on the engine increases and tends to slow the vehicle down. A cruise controller may respond to such a condition by increasing the fuel supply to the engine, and thus the power output of the engine, while maintaining a constant engine speed. The power output of the engine can increase up to a maximum power rating for the engine at any given speed. Some cruise control systems use engine droop to improve the fuel economy of the engine at high loads. This improvement is accomplished by ramping down or gradually reducing engine speed as the load on the engine increases past a predetermined level.
A typical implementation of droop curves for an engine governor can be seen in U.S. Pat. No. 5,868,214, which issued on Feb. 9, 1999 (the '214 patent). The '214 patent discloses a cruise control governor which is able to dynamically define and switch between various goal droop curves in order to find the best goal droop curve for use with a driving situation of a vehicle. Specifically, the '214 patent discloses a governor that is capable of increasing the torque generation of an engine when an engine underspeed is detected, and also capable of decreasing engine speed when the load is increasing to increase fuel economy of the vehicle. Such reduction of engine speed that is accompanied by an increase in torque is commonly referred to as “positive” or “standard” droop behavior, and is the norm in various applications.
An additional example of an engine governor using droop functionality can be seen in U.S. Pat. No. 5,553,589, which issued on Sep. 10, 1996 (the '589 patent). The '589 patent discloses a variable droop engine speed control system that includes a proportional-integral-derivative (PID) engine speed controller. The PID controller includes a droop gain that is only associated with the integral portion of the controller and that enables variation in the rate of droop applied to the engine under different conditions. Such droop is calculated dynamically in the controller during operation of the engine.